Slab Insulation/Rebar under Stove

[Kurt in MT]

I am preparing to pour a concrete slab for the detached sauna I am building. Concrete pouring weather here is still a few weeks away, so I am still in the planning stage. The sauna will be heated by a medium Kuuma sauna stove (about 400lbs). The stove is capable of heating the sauna room up to about 190F.

I have questions about the slab under the stove area (that section is approximately 6x8). I would prefer to use a 6 inch slab, with rebar and 2 inch rigid foam insulation underneath. My concern is about whether this could cause problems from the stove heat.

The rigid insulation I plan on using is rated for use at temperatures up to 165F. (Owens Corning Foamular). So, my first question is whether the heat generated from the stove would cause the bottom of the slab to exceed this level. I have no idea how to even approach this question.

The second question in my mind concerns the rebar. My instincts tell me that rebar will conduct heat, and may expend a bit. Would this process be likely to crack the slab?

Many thanks

Kurt

 

[bholler]

I am preparing to pour a concrete slab for the detached sauna I am building. Concrete pouring weather here is still a few weeks away, so I am still in the planning stage. The sauna will be heated by a medium Kuuma sauna stove (about 400lbs). The stove is capable of heating the sauna room up to about 190F.

I have questions about the slab under the stove area (that section is approximately 6x8). I would prefer to use a 6 inch slab, with rebar and 2 inch rigid foam insulation underneath. My concern is about whether this could cause problems from the stove heat.

The rigid insulation I plan on using is rated for use at temperatures up to 165F. (Owens Corning Foamular). So, my first question is whether the heat generated from the stove would cause the bottom of the slab to exceed this level. I have no idea how to even approach this question.

The second question in my mind concerns the rebar. My instincts tell me that rebar will conduct heat, and may expend a bit. Would this process be likely to crack the slab?

Many thanks

Kurt

What does the stove specify for hearth requirements? Kuma really is the only place to get your answer

 

[Kurt in MT]

What does the stove specify for hearth requirements? Kuma really is the only place to get your answer

Their materials require a non-combustible floor, that’s all they say. I know from reading other threads, that people use the rigid insulation under slabs with stoves on top. I just saw the heat rating on the spec sheet from Owens Corning, and it made me pause. On the rebar side, I just wondered if others might have used it under stoves and not had cracking.

 

[bholler]

Their materials require a non-combustible floor, that’s all they say. I know from reading other threads, that people use the rigid insulation under slabs with stoves on top. I just saw the heat rating on the spec sheet from Owens Corning, and it made me pause. On the rebar side, I just wondered if others might have used it under stoves and not had cracking.

I don't see any reason you would need rebar honestly. If you are questioning what hearth is required call kuma and ask them. They are the only ones that can really give you that answer

 

[Kurt in MT]

I don't see any reason you would need rebar honestly. If you are questioning what hearth is required call kuma and ask them. They are the only ones that can really give you that answer

I sent them an email asking about this (they are Kuuma, by Lamppa Mfg. out of MN (small operation making sauna stoves and wood furnaces), not the Kuma that makes residential wood stoves). They may well have no clue.

 

[bholler]

I sent them an email asking about this (they are Kuuma, by Lamppa Mfg. out of MN (small operation making sauna stoves and wood furnaces), not the Kuma that makes residential wood stoves). They may well have no clue.

If they don't know what floor is required under their stoves there is no way I would light a fire in it in my house. Is this a ul listed appliance? If so they know

 

[Highbeam]

The sauna may be up to 190 but do you expect that temperature at slab level? I would not expect any higher heat at foam level than any other sauna. Saunas are a niche area and I’d bet there is a forum for this. Using a wood heater is only slightly different.

I would not worry about the foam. There is no load on the slab middle. The edges are cooler where it supports the building. A 6” slab with bar is stout. Even if the foam in the middle melted, it would bridge it.

 

[Kurt in MT]

If they don't know what floor is required under their stoves there is no way I would light a fire in it in my house. Is this a ul listed appliance? If so they know

I hear you. This stove is designed for a standalone detached sauna, not for inside a home. They basically require a non-combustible floor (slab in diagrams) at least 16 inches larger than the base.

 

[Kurt in MT]

The sauna may be up to 190 but do you expect that temperature at slab level? I would not expect any higher heat at foam level than any other sauna. Saunas are a niche area and I’d bet there is a forum for this. Using a wood heater is only slightly different.

I would not worry about the foam. There is no load on the slab middle. The edges are cooler where it supports the building. A 6” slab with bar is stout. Even if the foam in the middle melted, it would bridge it.

Thanks much. The concrete/rebar is very stout for this small building application (live in a somewhat seismic area not far from Yellowstone). Your point about the bridge is a good one, hard to imagine the concrete failing.

 

[Highbeam]

Thanks much. The concrete/rebar is very stout for this small building application (live in a somewhat seismic area not far from Yellowstone). Your point about the bridge is a good one, hard to imagine the concrete failing.

I put 2” of 20psi foam under my 5” thick, 1800 sf slab. That 20 psi foam, like yours, is very strong. Stronger than almost all sub grade soils beneath. I seriously doubt it will see high temperatures. The concrete will distribute the heat across the surface and depth since it’s pretty conductive. Intermittently heated with wood, it will stay pretty cool. You might want a wood grid/deck to walk on.

Rebar never hurts.

 

[Rearscreen]

I think rebar is for building bridges. 42" x 7' remesh sheet from Home Depot is what you need.

 

[PA Mountain Man]

I went to Kuuma website to take a look around. They claim "If you’re looking for a durable, well constructed, approved to UL #1482 by PFS Laboratory, and an efficient woodburning sauna stove, our Kuuma meets your needs. "
The stove has grates and an ash pan, so I don't see the concrete getting too hot. The "non-combustible floor" requirement is the same as other wood stoves.
You will have to fire that sauna for a long time to heat up 6" of concrete to even be warm to walk on. No worries with the insulation or rebar.
Is this a floating slab, or do you plan on a foundation?

 

[Kurt in MT]

I put 2” of 20psi foam under my 5” thick, 1800 sf slab. That 20 psi foam, like yours, is very strong. Stronger than almost all sub grade soils beneath. I seriously doubt it will see high temperatures. The concrete will distribute the heat across the surface and depth since it’s pretty conductive. Intermittently heated with wood, it will stay pretty cool. You might want a wood grid/deck to walk on.

Rebar never hurts.

Excellent thoughts, thank you.

I am doing all this work myself, so I don’t have a concrete contractor to keep me out of trouble. It’s a big area for bags of concrete and a mixer (for one person), so I will be pouring the footers and floor in manageable sections. So, to cover up the sections (and my less than perfect finishing skills), I will cover the floor with duckboard sections done in cedar (except around the stove). The cedar will moderate the hot/cold floors (I am using fence pickets as my cedar raw material, plentiful and relatively cheap).

I really appreciate your input!

 

[Kurt in MT]

I went to Kuuma website to take a look around. They claim "If you’re looking for a durable, well constructed, approved to UL #1482 by PFS Laboratory, and an efficient woodburning sauna stove, our Kuuma meets your needs. "
The stove has grates and an ash pan, so I don't see the concrete getting too hot. The "non-combustible floor" requirement is the same as other wood stoves.
You will have to fire that sauna for a long time to heat up 6" of concrete to even be warm to walk on. No worries with the insulation or rebar.
Is this a floating slab, or do you plan on a foundation?

Yes, I know about the UL listing. Thanks so much for the insights.

The sauna stove will only run for a couple hours a session, so your point about heating up the floor is very insightful.

The construction is on a steep slope, dug in like a daylight basement. I am using footers and concrete block for the lower walls (half wall in back, one course of block in the front). The frostline here technically is very deep (36-48 inches), but the climate is very dry and the soil is extremely dry, stable and very highly compacted (glacial deposits of sand, gravel and rock). There only is a thin layer (a few inches) of topsoil....basically, it’s a high altitude semi desert. The foundations here don’t move significantly, except for occasional seismic events (which accounts for the rebar and relatively thick slab floor).

 

[Highbeam]

Wow. A 6” slab with bags. It’s going to feel like a million bags go in. Since you’re pouring footers I wonder if you couldn’t pave the inside with pavers. It’s a no load, floating floor, that you’re covering up after all.

 

[Kurt in MT]

Wow. A 6” slab with bags. It’s going to feel like a million bags go in. Since you’re pouring footers I wonder if you couldn’t pave the inside with pavers. It’s a no load, floating floor, that you’re covering up after all.

Interesting idea! I have poured quite a bit around the outside (walkways, patios) and you certainly start to feel the 80lb bags after a while. With the timing of the initial set (with a little delay additive), I can only work about 20 sq ft at a time (17-18 bags). Then wait three days to remove forms, do next section. So, it takes a while, but I am retired with time on my hands. Here are a few pics of the build site (been working on it most the winter, dry stone retains walls).

 

[begreen]

I'd go for a 4" slab, no rebar, using glass fiber, reinforced concrete instead. That is what our garage floor is (in an earthquake area). I have had a 4.5 ton RV parked in there and no issues.

 

[Kurt in MT]

Makes sense. I am not looking for extra work!